# Tag Info

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The answer to this is surprising: We are. And many (if not all) other galaxies. And they move faster than light. See, the universe is expanding, at an accelerating rate. The fabric of spacetime itself stretches out, so that galaxies seem to move away from each other. The interesting thing is that relativity does not forbid these from moving away faster ...

18

The answer to the headline question is: No. Most of Saturn's rings are below the Roche limit of about 2.5 Saturn radii. Hence tidal forces will prevent that part of the rings to form a (large) moon. Actually, part of the rings may be caused by loss of material from some of Saturn's moons, as suspected from observations of Enceladus. Accretion of Earth is ...

16

When an object is in orbit, there are two factors at play, not just one. The first, as you mention, is the force of gravity pulling the objects together. However, each object also has a momentum component which is generally (in the case of circular orbits) perpendicular to the direction of the gravity. If we look at the common situation of a small-mass ...

16

Each time Halley's comet passes us, we can make a pretty good estimate of its current orbit, and determine how close it will get to the massive bodies of the solar system like Jupiter, Saturn, Uranus, or Neptune on its next orbit. We can make good estimates of gravitational perturbation effects, and thereby know where to look for it. In fact, astronomers ...

14

There is also another mediator particle which moves at the speed of light other than the photon. This is the gluon, which is the exchange particle for the strong force. The odd thing about the gluon is that it's never seen by itself (that is, outside of collections of other gluons). Also, though neutrinos do in fact have mass, they are neutral particles. ...

14

The first question as stated has a rather trivial answer: "If the sun magically disappeared, instantly, along with all its influences, how long would it take its gravity to stop having an effect on us?" Since the Sun's gravity is among its influences, it would instantly stop having an effect on us. That's just part of the magical situation, and doesn't ...

13

Gravitational lensing is the effect that large amounts of gravity have on the path of light. Here's an example of the effect: The animation shows a black hole passing in front of a galaxy (simulated). In theory any object with a surface mass density larger than the critical surface mass density would be able to produce similar such effects. What's ...

13

Yes, the moon is moving away from Earth at around 1.48" per year. According to the BBC: The Moon is kept in orbit by the gravitational force that the Earth exerts on it, but the Moon also exerts a gravitational force on our planet and this causes the movement of the Earth's oceans to form a tidal bulge. Due to the rotation of the Earth, this tidal ...

13

The Hubble expansion has no bearing whatsoever on the length of the year. This is because the whole Milky Way galaxy (and in fact most galaxies, if not all, and even local groups) has decoupled from the Hubble flow long ago. In fact, it could only form after it decoupled. Note that M31, our sister galaxy, is in fact falling onto the Milky Way rather than ...

11

The tail of a comet is not actually "slowing down and falling away" from the comet, like you might expect to see when smoke streams out from behind a moving object on earth. The tail of a comet is actually being pushed away from the sun by the solar winds and radiation. That's why the tail of a comment always points away from the sun, and doesn't stream out ...

11

Absolutely possible. There's nothing magical about a black hole. The gravitational pull of a black hole reaches as far as gravity would for another object of the same mass. If you replace the Sun with a black hole of the same mass, everything would continue to orbit it just as it currently does. Anything with mass has a gravitational force itself, and a ...

11

There are plenty of rapidly moving objects in astrophysics. A good place where one can get moving relativistically is near an event horizon of a black hole. A simple Newtonian estimate illustrates the point. Black hole has all its mass $M$ hidden under an event horizon of the radius of order $r_{g}=\dfrac{2GM}{c^2}$. An object moving circularly in the ...

11

Hmmm no, it wouldn't be cluttered with debris, and yes, it's a good idea to park the JWST (James Webb Space Telescope) at the Sun-Earth L2 point. The five Lagrange points are unstable, for one because of the gravitational anomalies of the two massive bodies of the Lagrange system, eccentric orbits, and there are many other factors to their instability. At ...

11

Yes - this is the formula: $$F = G\frac{m_1m_2}{d^2}$$ Using this equation, we can say that all atoms in the universe exert force upon eachother. One carbon-12 atom has a mass of $1.660538921(73)\times10^{-27} kg$. That's a crazy small mass. Now let's say that these two atoms are 100,000,000 light years apart. That's $9.461\times10^{23} m$, which is a ...

11

Gravitational lensing is the bending of light by massive objects in between the observer (us), and a background source of light. It is a direct prediction of Einstein's theory of General Relativity, and was tested and confirmed by Sir Aurther Eddington during the famous Solar exlipse of May 29, 1919, where the apparent position of a star very close to the ...

11

Comet Shoemaker–Levy 9 crashed into Jupiter a few years back. As well as these molecules, emission from heavy atoms such as iron, magnesium and silicon was detected, with abundances consistent with what would be found in a cometary nucleus. Those heavy elements are consistent with the comet being at least being partially composed of rock. So Jupiter ...

11

The problem with trying to form a black hole with dark matter is that dark matter can only weakly interact (if at all) with normal matter and itself, other than by gravity. This poses a problem. To get dark matter concentrated enough to form a black hole requires it to increase its (negative) gravitational binding energy without at the same time increasing ...

10

There are other formulas at work, but not any other forces. You need to take into account ont only the force, thus the acceleration, but also the current velocity of a body orbiting another. To put it simply: if you move a ball sticked to a rope around your head, the only forces are the tension of the rope and gravity towards the floor. Ignoring gravity, ...

10

What is the difference between time and space-time? Space-time is time plus space. How does gravity affect the passage of time? The higher the gravity of a planet or star and the closer to that body the slower the time. What is the speed of light and how does it relate to time? The speed of light is 299,792.4580 km/s in vacuum, the speed at ...

9

(Disclaimer: As I already pointed out in a comment to the question above, I never did a calculation with $H_0$ before and I might be utterly, horrible wrong with my interpretation.) If you completely ignore the slowly changing orbit of earth and only take expansion of space into account and assume the Hubble-parameter to be pretty constant in the timeframe ...

9

Gravity doesn't affect the speed of light. It affects the space-time geometry and hence the paths of light. However, this can have a similar effect. Light emitted at source $S$ to pass a massive object $M$ that is very close on the otherwise (if M weren't there) straight path to an observer $O$ has to "go around" $M$, which takes longer than following the ...

9

As Walter says, gravity doesn't bend light. Light travels along null geodesics, a particular type of straight path. Since (affine) geodesics don't change direction by definition, geometrically light trajectories are straight. Moreover, the speed of light in vacuum is $c$ in every inertial frame, regardless of whether or not spacetime is curved, although a ...

9

The strength of the Earth's gravitational field compared to the Moon and the Sun is not enough to capture and hold satellites - there are too many disruptive forces that would rip them away over time. However there are some objects at the Lagrangian points - the points where the gravitational fields of the Earth and other objects are equal and so it is ...

9

In a car, you have a perception of speed because of (a) the "wind" passing by as you rush through the air which is not moving at the same speed as the vehicle, and (b) you perceive the stationary objects nearby as "moving" off into the distance behind. As the earth moves in its orbit, you don't notice any "wind" from the planet rushing through space, as the ...

9

Unless the stars comes so close that they actually collide, two stars will not be able to catch each other gravitationally. The reason is energy conservation: As they approach each other, their potential energy is converted into kinetic energy, increasing their velocities. When they are closest, their velocities are at their highest, but since there's ...

8

The currently leading answer is correct to say that moon formation inside the Roche limit is unlikely. However, the disk is evolving due to viscosity between the particles, and as a consequence it "spreads", so that material is able to move to outside the Roche limit. In fact this is a leading possible explanation for the formation of the inner moons of ...

8

(I will assume a Schwarzschild black hole for simplicity, but much of the following is morally the same for other black holes.) If you were to fall into a black hole, my understanding is that from your reference point, time would speed up (looking out to the rest of the universe), approaching infinity when approaching the event horizon. In ...

8

Any object with mass (even you) has gravity. The mutual attractive force between two objects is given by the formula $$F = G \frac{M_1 M_2}{R_{12}^2},$$ where the two mass are $M_1$ and $M_2$ and $R_{12}$ is the separation of their centres of mass. So to answer your question we need to define some sort of parameter that specifies what you mean by ...

7

There are certainly people who study alternative (non-General Relativistic) theories of gravity. The most popular theories have so far been: Modified Newtonian Dynamics (MOND) - which essentially postulates that Newtonian Mechanics break down on some scale, leading to the rotation curves we see in galaxies. Tensor–vector–scalar gravity (TeVeS) - this is a ...

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